In the construction of smoke detectors operating on the reflected light principle, a housing forming a dark chamber is provided with an internal light beam, and a photo-cell views the light beam in a suitable manner to respond only to light reflected from smoke particles in the light beam. Many detectors provide apertures leading into the chamber to permit ambient atmosphere to diffuse into the chamber. Since the aperture must provide a circuitous route for the ambient air, to prevent entrance of ambient light, the time required for ambient smoke to diffuse into the housing and reach the concentration required to actuate the alarm is longer than desirable.
To reduce the time required to get ambient smoke into the housing, means have been provided to continuously force smoke through the housing. In most installations, for reasons of economy and because of restrictions on the amount of power allowed to be drawn by the detector, the only practical means of accomplishing this is by the use of a small heating resistor, which is positioned in the detector to cause air flow therethrough by a chimney effect. The light and photo-cell are then positioned to respectively illuminate and view the column of smoke containing air passing through the detector.
However, it has been found that the response time of such a detector is very little, if any, faster than that of a detector that depends on the diffusion of ambient air into the chamber. The reason for this is apparently the fact that, due to the low power that induces the air flow, the resulting flow is laminar, rather than turbulent. Hence smoke entrained in the incoming air tends to remain stratified in layers. The illumination of the layers of smoke does not produce appreciable reflected light onto the photocell until the smoke concentration in the ambient atmosphere becomes unduly high.
Another problem encountered in detectors of this type in which the photocell and the light source are positioned in recesses in a block of material, said recesses being inclined toward each other and opening to the surface of the block adjacent each other, is the fact that a build-up of dust and lint on the surface between the apertures can allow the light to travel along the surface into the photo-cell aperture causing a false alarm.